In document copier machines of the electrophotographic type, charged latent images are produced on a photoreceptive material and developed through the application of a developer mix. A common type of developer mix is comprised of two components, a carrier material, such as a magnetic bead, coated with toner particles. Where the photoreceptive material is separate from the copy paper itself, a transfer of the developed image to the copy paper must take place together with fusing of the developed image to the paper. It is the toner that is attracted to the charged, latent image to develop that image and it is the toner that is transferred from the latent image to the copy paper and fused thereto to produce the finished copy.
It is apparent from the procedure outlined above that toner is a supply item which must be periodically replenished in the developer mix since toner is carried out of the machine on the copy paper as a reproduced image. It is also apparent that the concentration of toner particles in the developer mix is significant to good development of the latent image since too light a toner concentration will result in too light a developed image and too heavy a toner concentration will result in too dark a developed image.
Other variables which seriously affect copy quality include the image voltage of the photoconductor and the bias voltage on the developer. Many other variables factor into these basic quantities, for example, the quality of the original, the cleanliness of the optical system, and the condition of the photoconductor.
For a quality control system that attempts to accurately control toner concentration, image voltage, and other quality rendering factors, the control system itself must be designed to be as free from internal error as possible. Previous systems include U.S. Pat. No. 2,956,487 which provides a toner concentration control system where the reflectivity of the image to be reproduced is used as a measure of toner density. This system appears subject to difficulty since reflectivity readings will change dependent upon the quality of the original. U.S. Pat. No. 3,348,522 discloses a toner concentration control scheme in which a special test image is developed outside the image area used for reproducing document copies. In this latter patent, separate reflectivity-sensing devices are used to simultaneously sense light reflected from a single light source, one sensing device to establish a voltage indicative of clear photoconductor outside the image area and the other to establish a voltage indicative of the test area which, as noted above, is also outside the image area. U.S. Pat. No. 3,348,523 is essentially similar to U.S. Pat. No. 3,348,522.
U.S. Pat. No. 3,926,338 discloses a circuit for use in a toner concentration control scheme where a thermally insensitive photodetector is used to nullify the effect of the large amount of heat generated during machine operation. Similarly, this patent says that a stable amplifying circuit, stable referring to temperature stability, must be used in order to avoid destruction of the validity of the sensed signal.
U.S. patent application Ser. No. 141,864, filed Apr. 21, 1980, relates to an electrophotographic copier machine in which a test area is placed on a part of the photoconductor within the image area itself during a separate test cycle. In that manner, the advantages of using a developed image are combined with the advantages of using the very same photoconductor that is used for document reproductions. It was found that on short runs the test cycle could correspond to a run-out cycle after the last copy had been produced. However, during long, multicopy runs, it may be necessary to skip an occasional copy in order to provide a test cycle, once every 10 copies, for example. The test cycle technique improves accuracy since there is a tendency for toner to build up on the image area with photoconductor usage; since the photoconductor surface characteristics change with use, thus affecting development; and since the photoconductor suffers electrostatic degradation with use. A result of these factors is that the image area itself becomes darkened as compared to the areas of the photoconductor which are not used for image impressions and the photoconductor does not charge as well as it does when fresh. When photoconductor charge is reduced, the voltage levels of a resultant latent image are changed as compared to new photoconductor and copies are produced which are too light. However, in the test cycle technique, where the test area is produced within the image area any results of toner filming, aging, use, etc., are present in the quality tests. Consequently, the absolute quantity of toner in the developer mix can be adjusted as the photoconductor changes and the value of the developer bias voltage can be changed to provide compensating factors for the effects of change. Such results are not possible unless the quality tests are taken within the image area. Even if the tests are taken within the image area, there is still no assurance that the results will be accurate unless the testing circuit is able to compare the resulting quantities to a meaningful reference and unless the quantities are devoid of circuit-induced nonlinearities.
In the related patent named above, it was discovered that it is advantageous to view a cleaned, uncharged area of the photoconductor within the image area in order to provide a reference voltage subject to the variables named above. Additionally, it was discovered that various elemental factors such as temperature as well as component nonlinearities prevented accurate comparisons of reference voltage and sensed voltage unless the identical sensor is used for both measurements and unless it is excited to similar levels during both measurements. In this regard, it was discovered that the amount of light received for both sample and reference measurements by the sensor must be made equal (at the correct quality level) to avoid photodetector nonlinearities and a circuit arrangement to provide this property was invented.
In the system described in related U.S. Pat. No. 4,183,657, a reference voltage is obtained and allowed to vary from test to test by viewing an untoned "bare" area of the photoconductor. The fact that the reference voltage is sensed each time a test is made by the same photodetector used to sense the developed image provides an important advantage in minimizing the effect of variables associated with temperature, such as the effect of shifts in the magnitude of the dark current of the photodetector and shifts in the light output from the light source. Other factors such as changes in the optical characteristics of the photoconductor due to oxidation and surface changes are also minimized. As a consequence of this dynamism, the system becomes insensitive to temperature, becomes insensitive to variations in component qualities, and insensitive to other variables as noted.